Make re-running rebuild-identities a bit faster and add a little progress information

Summary:
Ref T13151. Ref T12164. Two small tweaks:

• If we aren't actually going to change anything, just skip the writes. This makes re-running/resuming a lot faster (~20x, locally).
• Print when we touch a commit so there's some kind of visible status.

This is just a small quality-of-life tweak that I wrote anyway while investigating T13152, and will make finishing off db024, db025 and db010 manually a little easier.

Test Plan:

• Set authorIdentityPHID + committerIdentityPHID to NULL.
• Ran rebuild-identities, saw status information.
• Ran rebuild-identiites again, saw it go faster with status information.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151, T12164

Differential Revision: https://secure.phabricator.com/D19484